Porth's Essentials of Pathophysiology, 4e

196

Integrative Body Functions

U N I T 2

of acid–base disorders: metabolic and respiratory (Table 8-9). Metabolic disorders produce an alteration in the serum HCO 3 – concentration and results from the addition or loss of nonvolatile acid or alkali to or from the extracellular fluids. A reduction in pH due to a decrease in HCO 3 – is called metabolic acidosis , and an elevation in pH due to increased HCO 3 – lev- els is called metabolic alkalosis . Respiratory disorders involve an alteration in the arterial PCO 2 , reflect- ing an increase or decrease in alveolar ventilation. Respiratory acidosis is characterized by a decrease in pH, reflecting a decrease in ventilation and an increase in PCO 2 . Respiratory alkalosis involves an increase in pH, resulting from an increase in alveolar ventilation and a decrease in PCO 2 . PrimaryVersus Compensatory Changes in pH Acidosis and alkalosis typically involve a primary or initiating event and a compensatory or adaptive state that results from homeostatic mechanisms that attempt to correct or prevent large changes in pH. For example, a person may have a primary metabolic acidosis as a result of overproduction of ketoacids and respiratory alkalosis because of a compensatory increase in ventila- tion (see Table 8-9). Compensatory mechanisms provide a means to control pH when correction is impossible or cannot be immediately achieved. Often, they are interim mea- sures that permit survival while the body attempts to correct the primary disorder. Compensation requires the use of mechanisms different from those that caused the primary disorder. For example, the lungs cannot

compensate for respiratory acidosis that is caused by lung disease, nor can the kidneys compensate for met- abolic acidosis that occurs because of chronic kidney disease. The body can, however, use renal mechanisms to compensate for respiratory-induced changes in pH, and it can use respiratory mechanisms to compensate for metabolically induced changes in acid–base balance. Because compensatory mechanisms become more effec- tive with time, there are often differences between the level of pH change that is present in acute and chronic acid–base disorders. SingleVersus Mixed Acid–Base Disorders Thus far acid–base disorders have been discussed as if they existed as a single primary disorder such as meta- bolic acidosis, accompanied by a predicted compensa- tory response (i.e., hyperventilation and respiratory alkalosis). It is not uncommon, however, for persons to present with more than one primary disorder or a mixed disorder. For example, a person may present with a low serum HCO 3 – concentration due to metabolic acidosis and a high PCO 2 due to chronic lung disease. Values for the predicted renal or respiratory compen- satory responses can be used in the diagnosis of these mixed acid–base disorders (see Table 8-9). 1–3,64,65 If the values for the compensatory response fall outside the predicted values, it can then be concluded that more than one disorder (i.e., a mixed disorder) is present. Since the respiratory response to changes in HCO 3 – occurs almost immediately, there is only one predicted compensatory response for primary metabolic acid–base disorders. This is in contrast to the primary respiratory disorders,

Summary of Single Acid–Base Disturbances andTheir Compensatory Responses

TABLE 8-9

Acid–Base Imbalance

Primary Disturbance

Respiratory Compensation and Predicted Response*

Renal Compensation and Predicted Response*, † ↑ H + excretion and ↑ HCO 3 – reabsorption if no renal disease ↓ H + excretion and ↓ HCO 3 – reabsorption if no renal disease ↑ H + excretion and ↑ HCO 3 – reabsorption Acute: 1 mm Hg ↑ PCO 2 →  0.1 mEq/L ↑ HCO 3 – Chronic: 1 mm Hg ↑ PCO 2 →  0.4 mEq/L ↑ HCO 3 – ↑ H + excretion and ↓ HCO 3 – reabsorption Acute: 1 mm Hg ↓ PCO 2 → 0.2 mEq/L ↓ HCO 3 – Chronic: 1 mm Hg ↓ PCO 2 →  0.4 mEq/L ↓ HCO 3 –

↓ plasma pH and HCO 3 –

↑ ventilation and ↓ PCO 2 1 mEq/L ↓ HCO 3 – → ↓ ventilation and ↑ PCO 2 1 mEq/L ↑ HCO 3 – → 0.25 to 1.0 ↑ PCO 2

Metabolic acidosis

1 to 1.5 mm Hg ↓ PCO 2

↑ plasma pH and HCO 3 –

Metabolic alkalosis

↓ plasma pH and ↑ PCO 2

Respiratory acidosis

None

↑ plasma pH and ↓ PCO 2

Respiratory alkalosis

None

Note: Predicted compensatory responses are in italics. *If blood values are the same as predicted compensatory values, a single acid–base disorder is present; if values are different, a mixed acid–base disorder is present. 12 † Acute renal compensation ≤ 48 hours, chronic renal compensation >48 hours. 12